Introduction to cloud computing Jiaheng Lu Department of Computer Science Renmin University of China www.jiahenglu.net.

Introduction to cloud computing Jiaheng Lu Department of Computer Science Renmin University of China www.jiahenglu.net

Cloud computing

Review:Why distributed systems? What are the advantages? distributed vs centralized? multi-servervs client-server? Geography Concurrency => Speed High-availability (if failures occur).

Review: What Should Be Distributed? Users and User Interface Thin client Processing Trim client Data Fat client Will discuss tradeoffs later Database Business Objects workflow Presentation

Review: Work Distribution Spectrum Presentation and plug-ins Workflow manages session & invokes objects Business objects Database Fat Thin Fat Thin Database Business Objects workflow Presentation

The Pattern: Three Tier Computing Clients do presentation, gather input Clients do some workflow (Xscript) Clients send high-level requests to ORB (Object Request Broker) ORB dispatches workflows and business objects -- proxies for client, orchestrate flows & queues Server-side workflow scripts call on distributed business objects to execute task Database Business Objects workflow Presentation

The Three Tiers Web Client HTML VB or Java Script Engine VB or Java Virt Machine VBscritpt JavaScrpt VB Java plug-ins Internet ORB HTTP+ DCOM Object server Pool Middleware ORB TP Monitor Web Server... DCOM (oleDB, ODBC,...) Object & Data server. LU6.2 IBM Legacy Gateways

Why Did Everyone Go To Three-Tier? Manageability Business rules must be with data Middleware operations tools Performance (scalability) Server resources are precious ORB dispatches requests to server pools Technology & Physics Put UI processing near user Put shared data processing near shared data Database Business Objects workflow Presentation

Google Cloud computing techniques

The Google File System

The Google File System (GFS) A scalable distributed file system for large distributed data intensive applications Multiple GFS clusters are currently deployed. The largest ones have: 1000+ storage nodes 300+ TeraBytes of disk storage heavily accessed by hundreds of clients on distinct machines

Introduction Shares many same goals as previous distributed file systems performance, scalability, reliability, etc GFS design has been driven by four key observation of Google application workloads and technological environment

Intro: Observations 1 1. Component failures are the norm constant monitoring, error detection, fault tolerance and automatic recovery are integral to the system 2. Huge files (by traditional standards) Multi GB files are common I/O operations and blocks sizes must be revisited

Intro: Observations 2 3. Most files are mutated by appending new data This is the focus of performance optimization and atomicity guarantees 4. Co-designing the applications and APIs benefits overall system by increasing flexibility

The Design Cluster consists of a single master and multiple chunkservers and is accessed by multiple clients

The Master Maintains all file system metadata. names space, access control info, file to chunk mappings, chunk (including replicas) location, etc. Periodically communicates with chunkservers in HeartBeat messages to give instructions and check state

The Master Helps make sophisticated chunk placement and replication decision, using global knowledge For reading and writing, client contacts Master to get chunk locations, then deals directly with chunkservers Master is not a bottleneck for reads/writes

Chunkservers Files are broken into chunks. Each chunk has a immutable globally unique 64-bit chunk- handle. handle is assigned by the master at chunk creation Chunk size is 64 MB Each chunk is replicated on 3 (default) servers

Clients Linked to apps using the file system API. Communicates with master and chunkservers for reading and writing Master interactions only for metadata Chunkserver interactions for data Only caches metadata information Data is too large to cache.

Chunk Locations Master does not keep a persistent record of locations of chunks and replicas. Polls chunkservers at startup, and when new chunkservers join/leave for this. Stays up to date by controlling placement of new chunks and through HeartBeat messages (when monitoring chunkservers)

Operation Log Record of all critical metadata changes Stored on Master and replicated on other machines Defines order of concurrent operations Changes not visible to clients until they propigate to all chunk replicas Also used to recover the file system state

System Interactions: Leases and Mutation Order Leases maintain a mutation order across all chunk replicas Master grants a lease to a replica, called the primary The primary choses the serial mutation order, and all replicas follow this order Minimizes management overhead for the Master

System Interactions: Leases and Mutation Order

Atomic Record Append Client specifies the data to write; GFS chooses and returns the offset it writes to and appends the data to each replica at least once Heavily used by Google ’ s Distributed applications. No need for a distributed lock manager GFS choses the offset, not the client

Atomic Record Append: How? Follows similar control flow as mutations Primary tells secondary replicas to append at the same offset as the primary If a replica append fails at any replica, it is retried by the client. So replicas of the same chunk may contain different data, including duplicates, whole or in part, of the same record

Atomic Record Append: How? GFS does not guarantee that all replicas are bitwise identical. Only guarantees that data is written at least once in an atomic unit. Data must be written at the same offset for all chunk replicas for success to be reported.

Replica Placement Placement policy maximizes data reliability and network bandwidth Spread replicas not only across machines, but also across racks Guards against machine failures, and racks getting damaged or going offline Reads for a chunk exploit aggregate bandwidth of multiple racks Writes have to flow through multiple racks tradeoff made willingly

Chunk creation created and placed by master. placed on chunkservers with below average disk utilization limit number of recent “ creations ” on a chunkserver with creations comes lots of writes

Detecting Stale Replicas Master has a chunk version number to distinguish up to date and stale replicas Increase version when granting a lease If a replica is not available, its version is not increased master detects stale replicas when a chunkservers report chunks and versions Remove stale replicas during garbage collection

Garbage collection When a client deletes a file, master logs it like other changes and changes filename to a hidden file. Master removes files hidden for longer than 3 days when scanning file system name space metadata is also erased During HeartBeat messages, the chunkservers send the master a subset of its chunks, and the master tells it which files have no metadata. Chunkserver removes these files on its own

Fault Tolerance: High Availability Fast recovery Master and chunkservers can restart in seconds Chunk Replication Master Replication “ shadow ” masters provide read-only access when primary master is down mutations not done until recorded on all master replicas

Fault Tolerance: Data Integrity Chunkservers use checksums to detect corrupt data Since replicas are not bitwise identical, chunkservers maintain their own checksums For reads, chunkserver verifies checksum before sending chunk Update checksums during writes

Introduction to MapReduce

MapReduce: Insight ”Consider the problem of counting the number of occurrences of each word in a large collection of documents” How would you do it in parallel ?

MapReduce Programming Model Inspired from map and reduce operations commonly used in functional programming languages like Lisp. Users implement interface of two primary methods: 1. Map: (key1, val1) → (key2, val2) 2. Reduce: (key2, [val2]) → [val3]

Map operation Map, a pure function, written by the user, takes an input key/value pair and produces a set of intermediate key/value pairs. e.g. (doc—id, doc-content) Draw an analogy to SQL, map can be visualized as group-by clause of an aggregate query.

Reduce operation On completion of map phase, all the intermediate values for a given output key are combined together into a list and given to a reducer. Can be visualized as aggregate function (e.g., average) that is computed over all the rows with the same group-by attribute.

Pseudo-code map(String input_key, String input_value): // input_key: document name // input_value: document contents for each word w in input_value: EmitIntermediate(w, "1"); reduce(String output_key, Iterator intermediate_values): // output_key: a word // output_values: a list of counts int result = 0; for each v in intermediate_values: result += ParseInt(v); Emit(AsString(result));

MapReduce: Execution overview

MapReduce: Example

MapReduce in Parallel: Example

MapReduce: Fault Tolerance Handled via re-execution of tasks. Task completion committed through master What happens if Mapper fails ? Re-execute completed + in-progress map tasks What happens if Reducer fails ? Re-execute in progress reduce tasks What happens if Master fails ? Potential trouble !!

MapReduce: Walk through of One more Application

MapReduce : PageRank PageRank models the behavior of a “random surfer”. C(t) is the out-degree of t, and (1-d) is a damping factor (random jump) The “random surfer” keeps clicking on successive links at random not taking content into consideration. Distributes its pages rank equally among all pages it links to. The dampening factor takes the surfer “getting bored” and typing arbitrary URL.

PageRank : Key Insights Effects at each iteration is local. i+1 th iteration depends only on i th iteration At iteration i, PageRank for individual nodes can be computed independently

PageRank using MapReduce Use Sparse matrix representation (M) Map each row of M to a list of PageRank “credit” to assign to out link neighbours. These prestige scores are reduced to a single PageRank value for a page by aggregating over them.

PageRank using MapReduce PageRank using MapReduce Map: distribute PageRank “credit” to link targets Reduce: gather up PageRank “credit” from multiple sources to compute new PageRank value Iterate until convergence Source of Image: Lin 2008

Phase 1: Process HTML Map task takes (URL, page-content) pairs and maps them to (URL, (PR init, list-of-urls)) PR init is the “seed” PageRank for URL list-of-urls contains all pages pointed to by URL Reduce task is just the identity function

Phase 2: PageRank Distribution Reduce task gets (URL, url_list) and many (URL, val) values Sum vals and fix up with d to get new PR Emit (URL, (new_rank, url_list)) Check for convergence using non parallel component

MapReduce: Some More Apps Distributed Grep. Count of URL Access Frequency. Clustering (K-means) Graph Algorithms. Indexing Systems MapReduce Programs In Google Source Tree

MapReduce: Extensions and similar apps PIG (Yahoo) Hadoop (Apache) DryadLinq (Microsoft)

Large Scale Systems Architecture using MapReduce User App MapReduce Distributed File Systems (GFS)

BigTable: A Distributed Storage System for Structured Data

Introduction BigTable is a distributed storage system for managing structured data. Designed to scale to a very large size Petabytes of data across thousands of servers Used for many Google projects Web indexing, Personalized Search, Google Earth, Google Analytics, Google Finance, … Flexible, high-performance solution for all of Google’s products

Motivation Lots of (semi-)structured data at Google URLs: Contents, crawl metadata, links, anchors, pagerank, … Per-user data: User preference settings, recent queries/search results, … Geographic locations: Physical entities (shops, restaurants, etc.), roads, satellite image data, user annotations, … Scale is large Billions of URLs, many versions/page (~20K/version) Hundreds of millions of users, thousands or q/sec 100TB+ of satellite image data

Why not just use commercial DB? Scale is too large for most commercial databases Even if it weren’t, cost would be very high Building internally means system can be applied across many projects for low incremental cost Low-level storage optimizations help performance significantly Much harder to do when running on top of a database layer

Goals Want asynchronous processes to be continuously updating different pieces of data Want access to most current data at any time Need to support: Very high read/write rates (millions of ops per second) Efficient scans over all or interesting subsets of data Efficient joins of large one-to-one and one-to-many datasets Often want to examine data changes over time E.g. Contents of a web page over multiple crawls

BigTable Distributed multi-level map Fault-tolerant, persistent Scalable Thousands of servers Terabytes of in-memory data Petabyte of disk-based data Millions of reads/writes per second, efficient scans Self-managing Servers can be added/removed dynamically Servers adjust to load imbalance

Building Blocks Building blocks: Google File System (GFS): Raw storage Scheduler: schedules jobs onto machines Lock service: distributed lock manager MapReduce: simplified large-scale data processing BigTable uses of building blocks: GFS: stores persistent data (SSTable file format for storage of data) Scheduler: schedules jobs involved in BigTable serving Lock service: master election, location bootstrapping Map Reduce: often used to read/write BigTable data

Basic Data Model A BigTable is a sparse, distributed persistent multi-dimensional sorted map (row, column, timestamp) -> cell contents Good match for most Google applications

WebTable Example Want to keep copy of a large collection of web pages and related information Use URLs as row keys Various aspects of web page as column names Store contents of web pages in the contents: column under the timestamps when they were fetched.

Rows Name is an arbitrary string Access to data in a row is atomic Row creation is implicit upon storing data Rows ordered lexicographically Rows close together lexicographically usually on one or a small number of machines

Rows (cont.) Reads of short row ranges are efficient and typically require communication with a small number of machines. Can exploit this property by selecting row keys so they get good locality for data access. Example: math.gatech.edu, math.uga.edu, phys.gatech.edu, phys.uga.edu VS edu.gatech.math, edu.gatech.phys, edu.uga.math, edu.uga.phys

Columns Columns have two-level name structure: family:optional_qualifier Column family Unit of access control Has associated type information Qualifier gives unbounded columns Additional levels of indexing, if desired

Timestamps Used to store different versions of data in a cell New writes default to current time, but timestamps for writes can also be set explicitly by clients Lookup options: “Return most recent K values” “Return all values in timestamp range (or all values)” Column families can be marked w/ attributes: “Only retain most recent K values in a cell” “Keep values until they are older than K seconds”

Implementation – Three Major Components Library linked into every client One master server Responsible for: Assigning tablets to tablet servers Detecting addition and expiration of tablet servers Balancing tablet-server load Garbage collection Many tablet servers Tablet servers handle read and write requests to its table Splits tablets that have grown too large

Implementation (cont.) Client data doesn’t move through master server. Clients communicate directly with tablet servers for reads and writes. Most clients never communicate with the master server, leaving it lightly loaded in practice.

Tablets Large tables broken into tablets at row boundaries Tablet holds contiguous range of rows Clients can often choose row keys to achieve locality Aim for ~100MB to 200MB of data per tablet Serving machine responsible for ~100 tablets Fast recovery: 100 machines each pick up 1 tablet for failed machine Fine-grained load balancing: Migrate tablets away from overloaded machine Master makes load-balancing decisions

Tablet Location Since tablets move around from server to server, given a row, how do clients find the right machine? Need to find tablet whose row range covers the target row

Tablet Assignment Each tablet is assigned to one tablet server at a time. Master server keeps track of the set of live tablet servers and current assignments of tablets to servers. Also keeps track of unassigned tablets. When a tablet is unassigned, master assigns the tablet to an tablet server with sufficient room.

API Metadata operations Create/delete tables, column families, change metadata Writes (atomic) Set(): write cells in a row DeleteCells(): delete cells in a row DeleteRow(): delete all cells in a row Reads Scanner: read arbitrary cells in a bigtable Each row read is atomic Can restrict returned rows to a particular range Can ask for just data from 1 row, all rows, etc. Can ask for all columns, just certain column families, or specific columns

Refinements: Locality Groups Can group multiple column families into a locality group Separate SSTable is created for each locality group in each tablet. Segregating columns families that are not typically accessed together enables more efficient reads. In WebTable, page metadata can be in one group and contents of the page in another group.

Refinements: Compression Many opportunities for compression Similar values in the same row/column at different timestamps Similar values in different columns Similar values across adjacent rows Two-pass custom compressions scheme First pass: compress long common strings across a large window Second pass: look for repetitions in small window Speed emphasized, but good space reduction (10-to-1)

Refinements: Bloom Filters Read operation has to read from disk when desired SSTable isn’t in memory Reduce number of accesses by specifying a Bloom filter. Allows us ask if an SSTable might contain data for a specified row/column pair. Small amount of memory for Bloom filters drastically reduces the number of disk seeks for read operations Use implies that most lookups for non-existent rows or columns do not need to touch disk